Solar collector

ABSTRACT

A solar collector panel is made from glass fiber reinforced concrete using a dissolvable core of polymer foam to form the internal passageways. The core is dissolved in a solution of solvent and polymer which impregnates and coats the concrete surfaces of the passageways to seal the passageways and to isolate the concrete from the heat transfer fluid.

This is a division of application Ser. No. 945,077, filed Sept. 22,1978, now U.S. Pat. No. 4,213,929 and a continuation-in-part of Ser. No.692,507, filed June 3, 1976, now U.S. Pat. No. 4,257,481.

BACKGROUND

The conversion of solar radiation into usable heat at adequatetemperatures is the subject of much interest, but, heretofore, has beenof relatively limited practical application. The basic technology is notnew; however, realization of that technology in the form of commercialhardware has been retarded by the cost of the hardware, and by the costof solar collector panels in particular. Most collector panels availabletoday use expensive materials, require sophisticated manufacturingtechniques available only in highly industrialized countries, and resultin an apparatus which often is aesthetically unacceptable. An efficient,attractive, very inexpensive collector which is easily made near or atthe sites of use, with low capital investment, and from cheap, commonmaterials by labor of minimum skill is essential to the growth ofpractical solar heating. The present invention provides such a solarcollector.

SUMMARY

Typical applications of the solar collector of the present invention areuse as the heat source for heating room air or domestic hot water, asthe energy source for room air cooling, for heating swimming pools, as aheat source for operating a vapor cycle engine to pump water or generateelectricity, and many other applications in which heated water or heatgenerated vapor pressure are desirable. The collector panels of thepresent invention are useable as roofing by themselves, or can be placedon existing roofs. The panels can be formed in colors, patterns, ortextures to closely resemble roof tiles or shingles. The panels can beused as paving for driveways, sidewalks, patios, swimming pool surroundsand the like. The panels can be used to form exterior walls.

According to the present invention, a solar collector panel is madeentirely from concrete reinforced with alkali resistant glass fiber toresult in an extraordinarily thin walled, light weight panel structureof great strength. Since the collector panel can be made in inexpensivemolds by hand or with fairly basic machinery, it can be made with laborof relatively low skill in non-industrialized areas. Little or no energyis required for the manufacture. The panels can be made at or near thesite of use.

A desire for relatively high collection efficiency has been a focus ofmuch of the prior design effort for collector panels. This desire hasbeen responsible in part for the complexity and cost of those panels.The desire for high efficiency is perhaps misguided, for the heat sourceis free and the only penalty of lower efficiency is the need for morecollector area; a penalty easily accomodated if the collector cost islow. Comparative testing of collector panels of the present inventionwith several different more complex, allegedly highly efficient priorart collector panels revealed that the panels of the present inventionwere far more efficient than the compared panels, thereby reducing,rather than increasing, the collector area requirements.

DETAILED DESCRIPTION

The following description is of a preferred embodiment of a solarcollector according to the present invention. The described embodimentis a flat collector suitable for use as a roofing panel. The materialsrequired are cement, sand, alkali resistant glass fiber, water, foamedpolymer sheet, and a solvent for the polymer. The equipment required isthat required to mix concrete, a suitable open mold, and hoses and astorage tank for the polymer solvent. The invention is preferablycarried out with apparatus for spray application of glass fiberreinforced concrete (GRC), which apparatus is well known and widelyused.

In the drawings;

FIG. 1 is a perspective view of a collector panel according to thepresent invention,

FIG. 2 is a cross-sectional view taken along line II--II of FIG. 1,

FIG. 3 is a plan view of the styrene foam core used in making the panelof FIG. 1,

FIG. 4 is a schematic view of the plumbing connections for an array of aplurality of the panels of FIG. 1,

FIG. 5 is a cross-sectional view taken along line V--V of FIG. 4,

FIG. 6 is an enlarged view of the circled zone of FIG. 5,

FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 4 andshowing a modification of the panel of FIG. 1, and

FIG. 8 is a perspective view of a dwelling having a roof mounted arrayof collector panels in accordance with the present invention.

FIGS. 1 and 2 show a collector panel 10 according to the presentinvention. The illustrated panel is of a flat surface configurationsuitable for use as a roofing panel or for application to an existingroof. The panel is on the order of one or two square meters in area andhas a thickness of one or two centimeters. Within the interior of thepanel is a pattern of conduits or passages comprising a plurality ofparallel ducts 12 which terminate at either end in header or manifoldchambers 14, 16. The concrete on the interior is impregnated and coatedwith a polymer to render the concrete impervious to water, gas, or otherfluid heat transfer media.

FIG. 4 shows the interconnection of a plurality of the panels of FIG. 1to form a collector array for a dwelling roof. The dotted arrow on theright side indicates the down slope direction. A heat transfer mediumsuch as water is circulated through inlet 18 of panel 41 to the lowermanifold 16, upwardly through the parallel ducts 12 to the uppermanifold 14 and thence through outlet 19. The heat transfer medium thencirculates through a "U" pipe 20 to the next panel 42 in similarfashion. The interconnections illustrated in FIG. 4 are aseries-parallel arrangement such that pairs of adjacent panels 41, 42are connected in series. Other pairs of panels 43 and 44, 45 and 46,etc. are connected in series and each such pair of panels is connectedin parallel with the other pairs of the array. The overall path ofcirculation is up slope to take advantage of natural connection.

FIGS. 5-7 show in greater detail the application of the collector panelsto a roof. The embodiment illustrated in FIGS. 5-7 is a modification ofthat of FIG. 1. As can be seen in FIGS. 5 and 6, the panels are adaptedto lap one another in shingle or clapboard fashion. As can be seen inFIG. 7, the flat central portion of the panel 10 is bordered by anintegral rim 70 which extends below the flat panel 10 to provide aninsulating air space below the panel and extends above the panel toprovide a rabett 72 to receive a sheet of glass 75 or transparentplastic. Although glass or plastic significantly impedes solarinsolation by reflection and by blocking radiation, particularly thatoutside the visible spectrum, glass reduces conduction and convectionlosses to the air. Where the ambient air temperature is low or wherewinds are strong, glass may be desirable to achieve adequatetemperatures. FIGS. 6 and 7 also show a lid 76 which covers the pipingin the space between panel arrays. A completed installation of collectorpanels on a roof is shown in FIG. 8.

The following description of the method of making solar collector panelsin accordance with the present invention is directed to the flat panelof FIG. 1, although the method is applicable to more complexconfigurations such as panels simulating roof shingles or terracottaroof tiles. The dimensions and materials are appropriate for a preferredembodiment. Although the method is described in the context of sprayapplication of the concrete, the method can be practiced by hand usinghand tools.

The panels are cast in an open mold which imparts the upper faceconfiguration of the panel. The mold can provide a textured surface orcan provide a simulation of shingles or tiles. The mold can be of anydurable material suitable for concrete casting. Preferably, the mold ismade of glass reinforced plastic or glass reinforced concrete (GRC). Toaid in release of the cast panel, the mold can be coated with aconventional concrete mold release agent by brush or spray.

When prepared, the mold is laid out on a horizontal surface and a layerof glass reinforced concrete (GRC) 3 or 4 mm. thick is sprayed into themold. Preferably, the concrete mix is about 3/4 cement by weight and 1/4sand by weight before the addition of water. At least 3% and preferablymore than 5% by weight of alkali resistant glass fiber, known as ARfiber, is added to the concrete. Such concrete reinforcement fiber isavailable from Pilkington Bros., Ltd. of England or Owens Corning of theUnited States. The spray apparatus simultaneously sprays the concreteslurry and projects the glass fibers chopped to the appropriate lengthof 2 to 10 cm. The first layer of GRC is vibrated or troweled with afloat to release air bubbles.

A core to form the passages or waterways is then placed on the firstlayer of GRC. The core 30 is illustrated in FIG. 3. The core is formedfrom a sheet of polymer foam such as polystyrene foam. A sheet ofpolystyrene foam approximately 3 or 4 mm. thick is cut in the patternillustrated by any appropriate technique such as a steel rule die, a hotwire cutter, or by hand using a template and knife. The patterncomprises bars 13 integral with end pieces 15, 17. The bars 13correspond to the ducts 12 of the finished panel and the end pieces 15,17 correspond to the manifold chambers 14, 16 of the panel. The endpiece 17 at the bottom of the panel is cut at an angle a to provide thelower manifold chamber with a slope for more complete drainage. Theinlet 18 and outlet 19 pipes are affixed to the foam and are therebyproperly positioned in the completed panel in communication with themanifold chambers 14, 16.

After the foam core 30 with the inlet and outlet pipes 18, 19 has beenpositioned in the mold over the first layer of GRC, concrete slurrywithout glass fiber is sprayed into the mold to fill all the spaces inand around the core 30 with concrete. The mold is again vibrated ortroweled with a float to release any air bubbles.

The casting is completed by the spray application of a final layer ofGRC 3 or 4 mm. thick and the mold vibrated or troweled to releaseentrapped air and to provide a smooth surface for the bottom face of thepanel.

The cast panel can be released from the mold after the concrete has set,usually the next day. The cast panel is then cured under very highhumidity conditions to develop maximum strength. The cure requires abouta week of a constant wet environment. The time for cure can be shortenedat elevated temperatures. For example, curing at 50° C. requires abouttwo days. When fully cured, the panels are dried to remove all excesswater. A week in the sun is sufficient.

When the panels are fully cured and thoroughly dried, the polymer corecan be removed. The core is dissolved in an industrial solvent for thepolymer. For polystyrene foam suitable solvents includeperchloroethylene, trichloroethylene, methyl ethyl ketone, methylisobutyl ketone, toluene, carbon tetrachloride, benzine, carbondisulphide, ethylene dichloride, methylene chloride, ethyl acetate, andothers. Preferably, the solvent is chosen on the basis of availability,cost, toxicity, flamability and low boiling point. Methylene chloride,Methyl ethyl ketone, or trichloroethylene are preferred for polystyrene.Because the density of polymer foams such as polystyrene foam is low,only a relatively small volume of polymer is present in a relativelylarge volume foam core. Consequently, dissolution of the foam is prompt.

The polymer core is dissolved by circulating the solvent through thepanel from a container and back to the container. Preferably a pressurehead of a meter or more is employed. The percentage of polymer in thesolvent solution increases as cores are dissolved. Preferably, thesolution contains a high percentage of dissolved polymer. Thepolymer-solvent solution wets and penetrates the dry concrete to depositthe polymer solution in the air bubbles and intersticies of the concretethereby impregnating and sealing the panel passageways against leakageor seepage of water. Further, some of the polymer solution remains as acoating on all of the interior waterway passages of the panel after thesolution has been drained from the panel. When so much polymer isdissolved that the viscosity of the solution becomes too great foradequate permeation, more solvent is added. The polymer coating hardensas the solvent evaporates and forms a barrier between the concrete andthe water or other heat transfer medium. Thus, the panel is protectedagainst leakage or seepage, and against leaching of the concrete bycirculating water. The water, in turn, does not contact the concrete andso does not become alkaline. The completed panel is impervious to water,antifreeze solutions, gases such as propane, butane, flurocarbons, andother fluid heat transfer media.

The solvent from the solution adhering to the passageways can beevaporated into the atmosphere or can be recovered in a closed aircirculation system by use of a simple condenser. When solvent isrecovered, the solution approaches a steady state ratio of solvent topolymer wherein the volume of polymer left in each panel approximatesthat brought to the solution by each core.

The completed panel can be painted or stained for appearance and toimprove heat absorbtion. Excess polymer solution may be used as avehicle for the paint, thereby utilizing all byproducts of the process.

I claim:
 1. A solar collector panel comprising a one-piece glass fiberreinforced concrete panel having internal passageways in the form of aplurality of parallel ducts which terminate at one end in an inletmanifold chamber and at the other end in an outlet manifold chamber, theinternal passageways being sealed with a hardened polymer coating, saidsolar collector panel including means to assist in drainage of the heattransfer medium, wherein the fiber reinforced concrete is made from amix comprising cement and sand, and in addition, includes at least 3% byweight alkalai resistant glass fiber.
 2. The solar collector panel ofclaim 1 wherein the polymer is polystyrene.